1. Field of the Invention
The present invention relates to a profiled rolling stock. More particularly, the present invention relates to rolling stock as a running rail or railroad track made of an iron-based alloy of carbon, silicon, manganese, chromium, elements that form special carbides and/or micro-alloy additives that influence the transformation behavior of the material, residual iron, and both standard and manufacture conditional impurities, with a cross section formed at least in part by accelerated cooling from the austenite region of the alloy. The present invention also relates to a process for producing profiled rolling stock having the above properties.
2. Background and Material Information
Rolling stock can be stressed in different ways based upon the field of use. Due to properties of the material, the highest individual stress places demands on the size of the component, which affects its longevity. For technical and economic reasons, adjusting the amount of material components to certain requirements can provide advantages according to the distinct individual stresses generated within a particular field of use. This is especially the case for a field of use in which different parts of the same component are subject to different stress levels.
Railroad tracks are an example of a metal unit that experiences different levels of stress. On the one hand, the top surface of the rails (the rail head) requires a high degree of wear resistance to support train wheels. On the other hand, due to bending stress in the track from the weight of train traffic, the track requires a high degree of strength, toughness, and fracture resistance in the remaining cross section.
To improve the service properties of the rails with increasing traffic and ever greater axle loads, many proposals have been made to increase rail head hardness.
For example, AT-399346-B discloses a process in which the rail head in the austenite phase of the alloy is dipped into, and then removed, from a coolant having a synthetic coolant additive until a surface temperature of the rail drops to between 450.degree. C. and 550.degree. C. This forms a fine pearlite structure with an increased material hardness. To carry out the process, EP 441166-A discloses a device that submerges the rail head into a basin that contains the appropriate coolant.
EP-186373-B1 shows another process for forming a stable pearlite structure in rails. A nozzle dispenses coolant to cool the rails. The distance between the nozzle and the rail head is a function of (1) the hardness value to be achieved for the rail head and (2) the carbon equivalent of the steel.
Examples of devices for carrying out this process for the heat treatment of profiled rolling stock, such as rails, are shown in (1) EP-693562-A, which discloses forming a fine pearlite structure with an increased hardness and abrasion resistance, and (2) EP-293002, which discloses producing a fine pearlitic structure in the rail head by cooling the rail head to 420.degree. C. with hot water jets followed with air jets.
EP-358362-A discloses a process in which the rail head is cooled rapidly from the austenite region of the alloy to a selected temperature above the martensite transformation point (the temperature at which the alloy transforms into martensite). After reaching the selected temperature, the cooling process levels off. The material undergoes a complete isothermic conversion into the lower pearlite phase to form a pearlite microstructure. According to the chemical composition of the steel, this transformation should occur without forming bainite.
EP-136613-A and DE-33 36 006-A teach producing a rail with a high wear resistance in the head and high fracture resistance in the foot. After rolling and air cooling, the rail is austenitized at 810.degree. C. to 890.degree. C. and cooled in an accelerated fashion. A fine pearlitic structure is produced in the region of the head and a martensitic structure is produced in the region of the foot, which is tempered afterwards.
According to these above prior art methods, a rolling stock for use in a railroad track with a high wear resistance in the head and a high strength and toughness in the remainder requires a fine pearlite structure. Further, an intermediary phase/bainite structure (possibly containing martensite) must be avoided.
Atoms diffuse during pearlite conversion. As the temperature drops, the speed of nucleation for the lamellar phases of carbide and ferrite increases, which forms the pearlite. This produces an increasingly fine pearlite structure that is stronger and more abrasion resistant. The pearlite formation therefore occurs via nucleation and growth, which the extent of the super-cooling and the diffusion speed determines, particularly for carbon and iron atoms.
If the cooling speed is further increased, or the conversion temperature is further decreased, carbon-containing, low-alloyed iron-based materials transform into a bainitic or an intermediary phase structure. It is hypothesized that in such an intermediary phase transformation (or bainite conversion) the fundamental lattice atoms are frozen and cannot diffuse. The structural transformation therefore occurs by shearing of the lattice. However, the smaller carbon atoms can still diffuse to form carbides. Such a structure, formed immediately below the temperature region of the conversion to fine lamellar pearlite (i.e., formed in the intermediary phase transformation), has a much coarser form. The carbides produced are markedly larger and disposed between the ferrite lamellas. This significantly degrades material toughness and material fatigue. The finished article is easier to fracture, particularly under abrupt stress. Consequently, rails should not contain any bainite content in the structure.
WO 96/22396 discloses a carbide-free bainitic steel with a high degree of wear resistance and improved contact fatigue resistance. A low-alloy steel with high silicon and/or aluminum contents of 1.0-3.0 wt. %, 0.05-0.5 wt. % carbon, 0.5-2.5 wt. % manganese, and 0.25-2.5 wt. % chromium, cooled continuously from the rolling temperature produces substantially carbide-free microstructure rolling stock of the "upper bainite" type. This "upper bainite structure type" is a mixed structure of bainitic ferrite, residual austenite, and high carbon martensite. However, at low temperatures and/or when there are mechanical stresses, at least part of the residual austenite in the structure can shear and form martensite and/or a so-called deformation martensite. This increases the danger of crack initiation, especially at the phase boundaries.
An increase in the advent of traffic on the rail segments and higher axle loads and train speeds in general require higher material qualities and should also be achieved through improved service properties of rails.
A drawback of the prior art rolling stock produced from low-alloyed iron-based materials, and the associated processes (particularly heat treatment processes) for producing rolling stock with improved service properties, is that a further increase in the wear resistance and strength of the material can only be achieved through expensive technical alloying measures.